Summer black stem of alfalfa: I. Screening and isolate x genotype interactions. II. Molecular characterization of Cercospora spp. III. Registration of KS-2000-SBS (Cercospora medicaginis, Medicago sativa)

Summer black stem, incited by Cercospora medicaginis (Ellis and Everh.) is a common alfalfa (Medicago sativa L.) disease. Low levels of genetic resistance to this disease are present in alfalfa varieties. The objectives of this research were to; refine a population screening technique as a means of identifying moderately resistant germplasm, classify the extent to which Cercospora isolates differ in virulence patterns and aggressiveness, study levels of genetic diversity among isolates of Cercospora with Amplified Fragment Length Polymorphisms (AFLP's) and, improve levels of resistance in a moderately resistant germplasm source via recurrent selection. Data collection with the population screening method began six days after inoculation and occurred every second day thereafter for six days. Five identical tests were conducted. Entry, test, and entry x test mean differences were statistically significant (P < 0.05), however the most resistant and susceptible entries did not change rank. A single data collection event, 12 days after inoculation, will provide useful results. Latent period duration was explored as a means of identifying virulence and aggressiveness differences among 8 fungal isolates. Results revealed that all isolates were statistically similar. Understanding the genetic diversity of Cercospora medicaginis, and other Cercosporae that attack forage legumes, will aid researchers in preventing yield loss by including predominant isolates in screening programs. AFLP analysis of 926 bands revealed that C. medicaginis is primarily composed of one genetic lineage and that infection of alfalfa by other species of Cercospora occurs, but is not common. Rare isolates were detected and should be included in screening programs. Latent period duration of KS-211, a moderately resistant germplasm source, was increased through recurrent selection. Initially, 800 plants were screened and one-hundred plants were chosen as parents based on latent period duration. Cycle 2 seed was produced by forming 156 half-sib families. Up to six seedlings of each family were screened for latent period duration. Combined selection was used for the identification of parent plants for cycle 3 seed. Ninety three individuals were selected. Cycle 3 seed will be available in the spring of 2001. The results of these research projects should prove useful in decreasing losses by Cercospora medicaginis.